1. CSC1401 Classes - 1

2. Learning Goals Computing concepts Identifying objects and classes Declaring a class Declaring fields Default field values

3. Objects and Classes We have been using them all semester Picture beachscene = new Picture(“beach.jpg”); A big advantage of the picture class was that any method we wrote could be used in any program that had a picture object! We will explore how to create a class from scratch, rather than modifying an existing class

4. Identifying Objects and Classes Object-oriented programs Consist of interacting objects Which are defined by and created by classes To identify the objects in a task What are the things that are doing the work or being acted upon? How do you classify them? What data (fields) do they need to know to do the task? What procedures (methods) do they need?

5. Identifying the Objects and Classes Say that we want to write a program to do a slide show A series of pictures shown one after the other with some time waiting between the pictures One way to start is to underline the nouns Slide show, picture, wait time A slide show has pictures and a time to wait between pictures

6. How to create a slide show? We can use an array of pictures Then, we can loop through the pictures, displaying each picture

7. An array of Pictures Recall how we had arrays of Pixels (when first examining pictures We can do the same thing here: Picture [] pictureList;

8. An array of pictures – allocating memory To have the picture list contain 5 pictures: pictureList = new Picture [5]; To set the first element in the picture list to be the beach scene: pictureList[0] = new Picture (“beach.jpg”);

9. The resulting code public static void main(String[] args) { Picture[] pictureList = new Picture[5]; pictureList[0] = new Picture(“23beach.jpg"); pictureList[1] = new Picture(“23blueShrub.jpg"); pictureList[2] = new Picture(“23church.jpg"); pictureList[3] = new Picture(“23redDoor.jpg"); pictureList[4] = new Picture(“23butterfly.jpg"); int index; for (index = 0; index < 5; index ++) { pictureList[index].show(); } }

10. A problem All 5 pictures show up at once In a slide show, the pictures are supposed to show up one at a time, and there is a brief pause to wait while the picture is being displayed

11. Causing a wait for a Slide Show Use Thread.sleep(waitTime) to wait for waitTime number of milliseconds For wait time we can use integer to hold the number of milliseconds to wait This can cause an exception so write the method to throw Exception by adding throw Exception

12. The modified code public static void main(String[] args) throws Exception { Picture[] pictureList = new Picture[5]; pictureList[0] = new Picture(“23beach.jpg"); pictureList[1] = new Picture(“23blueShrub.jpg"); pictureList[2] = new Picture(“23church.jpg"); pictureList[3] = new Picture(“23redDoor.jpg"); pictureList[4] = new Picture(“23butterfly.jpg"); int index; for (index = 0; index < 5; index ++) { pictureList[index].show(); Thread.sleep(2000); pictureList[index].hide(); } }

13. Creating a class This slide show is neat, but I can only use it in the current program I’d like to be able to reuse it, like we did with the methods in the Picture class So, let’s create a slide show class!

14. Class Definition Each class is defined in a file With the same name as the class: SlideShow.java Class names Are singular (SlideShow not SlideShows) Start with an uppercase letter The rest of the word is lowercase Uppercase the first letter of each additional word The syntax for a class definition is: visibility class Name {} Inside the class definition goes: Fields, constructors, and methods

15. Class Declaration To declare a SlideShow class Click on the File-New-Java button in jGRASP Type in: public class SlideShow { } Save it in SlideShow.java Click on File then Save Compile the file

16. Class contents Classes contain variables (state) and methods We have been writing methods for classes (e.g. the Picture class) What variables might our class need? Two different kinds of variables within a class: Local to a method Class-wide

17. Declaring Fields Syntax visiblity type name; visibility type name = expression; Usually use private for the visibility So that other classes can’t access it directly The type is any of the primitive types, a class name, or an interface name Arrays are declared with [] after the type or after the name type[] name; or type name[]; Names start with a lowercase letter The first letter of each additional word is uppercased

18. Default Field Values If you don’t specify an initial value for a field It will get one anyway when it is created Numbers = 0 Objects = null (not referring to any object yet) boolean = false public class SlideShow { //////////////// fields /////////////////////////////////////////// private Picture[] pictureArray; private int waitTime; } Initial value will be null

19. Testing the SlideShow Class Add the fields to the class definition and compile it Try the following SlideShow slideShowObj = new SlideShow(); Don’t worry, the class isn’t supposed to do anything yet!

20. Next We need to decide what methods/functions this class will have, and to write them

21. All Classes Inherit from Object If you don’t specify the parent class when you declare a class The class with inherit from java.lang.Object You can specify the parent class Add extends Parent to the class declaration public class SlideShow extends Object A declaration of public class SlideShow Is the same as public class SlideShow extends Object

22. Summary Object-oriented programs Have interacting objects To decide what classes to create Identify the objects doing the action or being acted upon And classify them (what type of thing are they?) To declare a class public class SlideShow{} To declare a field private type fieldName;

23. Assignment Read Media Computation Chapter 11, Sections 1-2